1. Field
The embodiments described herein relate to organic light-emitting diodes, such as organic light-emitting diodes comprising a hole-transport layer, two light-emitting layers, and an electron-transport layer.
2. Description of the Related Art
White organic light emitting devices (WOLED) are becoming increasingly important for lighting applications. For example, WOLEDs may be able to replace fluorescent tubes to save energy. Thus there is a continuing need to improving the power efficiency of WOLEDs.
Many of the current WOLEDs comprise a hole-transport layer, at least two light-emitting layers, and an electron-transport layer arranged in that order. In these devices, each layer has a highest occupied molecular orbital (HOMO) energy level and a lowest unoccupied molecular orbital (LUMO) energy level, wherein the HOMO energy levels and/or the LUMO energy levels decrease in a stepwise fashion. In other words, with the energy levels of the first light-emitting layer are lower than the corresponding energy levels of the hole-transport layer (e.g. the HOMO of first light-emitting layer is lower than the HOMO of the hole-transport layer, the LUMO of the first light-emitting layer is lower than the LUMO of the hole-transport layer), the energy levels of the second light-emitting layer are lower than the corresponding energy levels of the first light-emitting layer, and the energy levels of the electron-transport layer are lower than the corresponding energy levels of the second light-emitting layer. These devices may suffer from the problems of electron leakage from the light-emitting layers through the hole-transport layer to the anode, and hole leakage from the light-emitting layers through the electron-transport layer to the cathode, thus reducing the device efficiency. Traditionally, hole-blocking and electron-blocking layers have sometimes been used to attempt to address this problem, but the additional layers add expense and complexity to the device fabrication and may reduce device efficiency.
Other devices may utilize hole-transport layers with very high LUMOs to block electron leakage to the anode an/or electron-transport layers with very low HOMOs to block hole leakage to the cathode. Unfortunately, the large energy gap between the corresponding molecular orbital of the hole-transport layer or the electron-transport layer and the corresponding electrode can significantly reduce hole or electron mobility. The reduced mobility can in turn cause reduced efficiency of the device. The large energy gap may also cause higher driving voltage. Thus, it is difficult to improve efficiency using this approach.
Thus, additional options for addressing these problems are needed.